Technical Field
The present invention relates generally to the field of flat panel displays, and more specifically, but not exclusively, to an improved Active Matrix Organic Light Emitting Diode (AM OLED) display and method of wide dynamic range dimming in such a display for commercial and military applications, such as, for example, cockpit displays, avionics displays, or hand-held military communication device displays.
Description of Related Art
AM OLED displays are an emerging flat panel display technology, which has already produced such new products as passive matrix-addressed displays that can be used for cell-phones and automobile audio systems. AM OLED displays are most likely to replace backlit AM Liquid Crystal Displays (LCDs) because AM OLED displays are more power efficient, rugged, weigh less, cost less, and have much better image quality than existing AM LCDs. As such, the market for AM OLED-based displays is estimated to reach about $1.7 B per year by 2006.
Cockpit display applications are relatively demanding for existing display technologies, because of the stringent requirements imposed with respect to image quality and the need for superior operational performance within a broad range of environments, such as high temperature, humidity, and ambient lighting environments. For the better part of the past ten years, AM LCDs have replaced Cathode Ray Tube (CRT) displays in cockpit applications, because of the advantages of AM LCDs over CRT displays in terms of lower weight, flatter form factor, less power consumption, the use of large active areas with relatively small bezels, higher reliability, higher luminance, greater luminance uniformity, wider dimming range, and better sunlight readability. As such, AM LCDs have been the displays of choice for cockpit and avionics display applications for a number of years.
A significant problem that exists with AM LCDs for display applications (e.g., cockpit, avionics and hand-held device displays) is that the backlighting of the AM LCDs adds a significant amount of weight and volume to these types of displays. However, an advantage of this backlighting feature of AM LCDs is that it provides a highly controllable function for (independently) dimming the display in order to achieve optimum performance over a range of ambient lighting conditions. Some critical display applications (e.g., avionics and certain military device displays) require wide dynamic ranges of dimming (e.g., >2000:1) for the display to be viewed comfortably in both daytime (bright) and night-time (dark) viewing conditions. Currently, this dimming function can be accomplished with AM LCDs by dimming the display backlight (through a large dynamic range), while maintaining the AM LCD's optimized driving conditions.
The weight and volume problems that exist with AM LCDs for avionics or hand-held device applications, for example, can be alleviated with AM OLED displays. Compared to AM LCDs, AM OLED displays offer such significant advantages as wider viewing angles, lower power consumption, lighter weight, superior response time, superior image quality, and lower cost. However, a drawback of the existing AM OLED displays is that they are not easily dimmable (i.e., their brightness adjusted) to the desired luminance levels, except by changing the driving conditions of the AM OLED displays, or by varying the anode (VDD) and/or cathode (VK) voltages.
Generally, the existing AM OLED displays' grayscale driving conditions are optimized for “normal” daytime (bright ambient) viewing conditions. However, changing either the grayscale driving conditions or the VDD/VK voltages of AM OLED displays to achieve lower display luminance levels for night (dark ambient) conditions using a conventional AM OLED display results in luminance and color non-uniformities across the surfaces of these displays.
As such, an important requirement imposed on AM OLED displays in such critical applications as cockpit displays, avionics displays, or military hand-held device displays is that such displays have to be capable of adjusting their luminance (brightness) over a wide dynamic range (e.g., >2000:1) without affecting the color balance and/or the uniformity of the luminance and chromaticity across the surface of the display as the display is being dimmed. The drive methods used for existing AM OLED displays achieve the desired luminance by adjusting the grayscale data voltage (or current) or VDD/VK voltage(s). However, these existing methods of adjusting the luminance of AM OLED displays create numerous problems for wide dynamic range display dimming applications, such as: (1) it is a relatively difficult problem to achieve the desired wide dynamic range dimming requirements with the existing driving methods using 8-bit data (column) drivers currently available for AM OLED displays; (2) when the grayscale data voltages (or currents) or the VDD/VK voltages, which are optimized for “normal” daylight operation, are changed (e.g., reduced) for night-time (low luminance) operation, typically the display color balance is changed due to the different transfer characteristics (luminance versus voltage) for the Red, Green and Blue (R, G, B) AM OLED display materials used; and (3) operation of the existing AM OLED displays at the low luminance levels associated with night-time viewing conditions results in significant non-uniformities in the luminance and chromaticity across the surface of the displays due to increased variations in the Thin-Film Transistor (TFT) and OLED performance in the low luminance (gray-level) regime.
As such, to illustrate these problems with existing AM OLED displays, FIG. 1 depicts an electrical schematic diagram of a typical AM OLED sub-pixel circuit 100 (labeled “Prior Art”), which is currently used in a conventional method for dimming an AM OLED display. Referring to FIG. 1, conventional sub-pixel circuit 100 includes a first TFT 102, a second TFT 104, a storage capacitor 106, and an OLED pixel 108. As shown, transistor 102 is a scan transistor, and transistor 104 is a drive transistor. The gate terminal 110 of the scan transistor 102 is connected to the row (scan/row enable) address bus of the display involved, and the drain terminal 112 of scan transistor 102 is connected to the column (data) address bus of the display. The source of scan transistor 102 is connected to the node 107 at the storage capacitor 106 and the gate terminal of the drive transistor 104. During the row addressing time period of the display operation, scan transistor 102 charges the node 107 at the storage capacitor 106 and the gate terminal of the drive transistor 104 to the data voltage (signal), VDATA. After the row addressing time period, scan transistor 102 is switched off, and the OLED pixel 108 is electrically isolated from the data bus. During the remainder of the frame time, the power supply voltage, VDD, which is connected to the drain terminal 114 of the drive transistor 104, provides the current for driving the OLED pixel 108.
The grayscale from this conventional method in the AM OLED display circuit 100 depicted in FIG. 1 is achieved by varying the data voltages (signals) on the data bus. In addition, the brightness (maximum luminance) of the display is adjusted (for display dimming) directly by changing the data voltages (signals) or VDD/VK voltages. However, as discussed earlier, it can be seen from FIG. 1 that a significant problem with these conventional methods of adjusting the luminance of an AM OLED display is that because the dimming is performed by changing the data voltage (or current), or by changing the power supply (VDD and/or VK) voltages to adjust the grayscale, wide dynamic range dimming (e.g., >2000:1) cannot be achieved with suitable uniformity. Nevertheless, as described in detail below, the present invention provides an improved AM OLED display and method of adjusting luminance with superior dimming capability (e.g., wide dynamic range >2000:1) that resolves the problems encountered with existing AM OLED displays and other prior art displays.